1. Field of the Invention
This invention relates generally to a portable transceiver. More particularly, the invention relates to generating amplitude matched 45 degree (45°) phase separated local oscillator signals.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld, telephone-like communication handset, also referred to as a portable transceiver. The different modulation and transmission schemes each have advantages and disadvantages.
As these mobile communication systems have been developed and deployed, many different standards have evolved, to which these systems must conform. For example, in the United States, many portable communications systems comply with the IS-136 standard, which requires the use of a particular modulation scheme and access format. In the case of IS-136, the modulation scheme is narrow band offset π/4 differential quadrature phase shift keying (π/4-DQPSK), and the access format is TDMA.
In Europe and emerging elsewhere, the global system for mobile communications (GSM) standard requires the use of the gaussian minimum shift keying (GMSK) modulation scheme in a narrow band TDMA access environment, which uses a constant envelope modulation methodology.
Portable transceivers operating in these various systems have been migrating from a methodology in which a baseband signal carrying data is first upconverted to an intermediate frequency (IF) prior to upconversion to radio frequency (RF) signal level at which the signal is transmitted to a system in which the data carrying baseband signal is upconverted directly to the RF frequency. Similarly, instead of being downconverted to an IF signal prior to being downconverted to baseband, many new portable transceivers downconvert a received RF signal directly to baseband. Such a portable transceiver is generally referred to as a “direct conversion receiver” and has many advantages over a portable transceiver that employs conversion to an IF signal as part of the transmit and receive signal processing.
Generally, in a direct conversion receiver, an oscillator supplies what is referred to as a “local oscillator” or “LO” signal to a mixer. The mixer downconverts the received RF signal to a baseband signal determined by the frequency of the local oscillator signal.
Some direct conversion receivers employ what is referred to as a “subharmonic mixer” to downconvert the received RF signal directly to a baseband signal from which the data contained in the signal may be reliably extracted. Conventional direct downconversion systems typically generate in-phase (I) and quadrature (Q) outputs using signals that are separated in phase by 90°. Because a subharmonic mixer effectively performs a multiply-by-two to the path containing the LO signal, the subharmonic mixer typically requires signals that are separated in phase by 45 degrees (°). Unfortunately, accurately generating the 45° offset phase signals has proven troublesome.
In one prior solution, a 90° phase shift inducing element is introduced into the RF signal path instead of requiring 45° separated LO signals. Unfortunately, this solution degrades the performance of the receiver by raising the noise figure. Another prior solution used a delay locked loop (DLL) to generate the 45° phase shifted signals. In a DLL, the LO signal drives a mixer and a buffer chain. The output of the mixer is integrated and the delay through the buffer chain is adjusted based on the output of the integrator. Unfortunately, a DLL adds significant complexity and consumes a large amount of power.
Therefore it would be desirable to efficiently and accurately generate the 45° phase shifted LO signals to supply to a subharmonic mixer.